The present invention relates to a process for electrochemically modifying aluminum or aluminum-alloy-based support materials for printing plates, which have been grained in a multi-stage process, after the first mechanical and/or the second electrochemical graining stage, and to the use of the materials thus modified in the manufacture of offset-printing plates.
Support materials for offset-printing plates are provided, on one or both sides, with a radiation-sensitive (light-sensitive) layer (copying or reproduction layer), either directly by the user or by the manufacturer of precoated printing plates, this layer permitting the production of a printing image (printing form) by a photomechanical process. Following the production of the printing form, the layer support carries the printing image-areas and, simultaneously, forms, in the areas which are free from an image (non-image areas), the hydrophilic image-background for the lithographic printing operation.
The following requirements must, among others, be met by a support for a radiation-sensitive material of this type used in the manufacture of lithographic plates:
Those portions of the radiation-sensitive layer which are comparatively more soluble following irradiation must be capable of being easily removed from the support, by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue.
The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink.
The photosensitive layer must exhibit an adequate degree of adhesion prior to exposure, and those portions of the layer which print must exhibit adequate adhesion following exposure.
The support material should possess a good mechanical stability, for example, against abrasion and a good chemical resistance, particularly with respect to alkaline media.
Water requirements during printing should be as low as possible, for example, to prevent excessive moistening of the paper because, otherwise, "register difficulties" in color printing (i.e., the second or third color shade can no longer be printed in register upon the first color shade) or breaks in the paper web in rotary offset printing may occur.
In order to meet some of these requirements, support materials of aluminum, which are conventionally employed in practice, are first subjected to a mechanical, chemical and/or electrochemical graining treatment, which additionally may be followed by an anodic oxidation of the grained aluminum surface. Particularly electrochemically grained aluminum surfaces with their very fine-grained structure forming an interface between the support material and the radiation-sensitive layer of printing plates, produce, in the printing forms which can be manufactured from these plates, results which meet practical requirements and which already comply with most of the demands. Water requirements during printing are, however, often still too high in the support materials which have been grained and optionally anodically oxidized according to known processes. Modifications of these processes have, therefore, already been described which especially may be applied after the graining stage.
German Offenlegungsschrift No. 30 09 103 (equivalent to South African Pat. No. 81/1545) discloses an abrasive modification of electrochemically grained support materials for printing plates comprising aluminum. In this modifying treatment removal of material from the surface in the order of from 0.4 to 3.0 g/m.sup.2 is effected under the action of an aqueous-alkaline solution which has a pH exceeding 11. Printing plates manufactured from support materials which have been thus modified and optionally anodically oxidized, are stated to have a lower consumption of dampening solution and a reduced adsorptivity.
In the method of producing printing plate support materials of aluminum according to German Offenlegungsschrift No. 25 57 222 (similar in content to U.S. Pat. No. 3,935,080), the support material is additionally cathodically modified (cleaned) in an aqueous sulfuric acid, between the stage of electrochemical graining in an aqueous hydrochloric acid and the stage of anodic oxidation in an aqueous sulfuric acid. It is stated that the method is, in the first place, suitable for use in a continuous process and that it results in a very clean surface.
From the prior art, a cathodic treatment is also known for use in other methods. For example, according to German Auslegeschrift No. 24 20 704 (equivalent to U.S. Pat. No. 3,865,700) cathodic contacting of aluminum supports is used in the anodic oxidation of these supports in an aqueous sulfuric acid, in order to prevent the use of contact rolls, which are normally present. German Pat. No. 25 37 724 (equivalent to British Pat. No. 1,532,303) discloses a one-stage graining process without subsequent abrasive modification of the surface, in which aluminum support-materials for printing plates are electrochemically treated in agitated aqueous salt solutions having a salt concentration of at least 200 g/l a pH ranging from 5 to 8 and a temperature of less than 60.degree. C. The salts used are alkali metal salts, alkaline earth metal salts or ammonium salts of hydrohalogenic acids or oxo-acids of nitrogen or of halogens. In a process variant (resulting in surfaces of type A), the aluminum can be grained, in a cathodic circuit arrangement, for a period of from 30 to 60 seconds with direct current of 70 to 150 A/dm.sup.2, whereby a silvery surface with a dull finish is produced; in this variant, alkali metal salts are exclusively used. Similarly, German Pat. No. 25 37 725 (equivalent to British Pat. No. 1,532,304) describes a possible cathodic circuit arrangement for the graining of aluminum, in which the aqueous electrolyte, at a pH ranging from 1 to 5, must contain an alkali metal salt in addition to aluminum salts.
The use of multi-stage processes for the purpose of graining support materials for lithographic printing plates is disclosed, for example, in U.S. Pat. Nos. 2,344,510; 3,929,591 and 4,477,317 (equivalent to British Pat. No. 1,582,620), in Japanese Patent applications Nos. 123,204/78; 145,701/78; 16,918/82 and 197,090/83, in German Offenlegungsschriften Nos. 30 12 135 (equivalent to U.S. Pat. No. 4,476,006 or British patent application No. 2,047,274), 30 31 764 (equivalent to British patent application No. 2,058,136) and 30 36 174 (equivalent to British patent application No. 2,060,923) and in European patent application No. 0,131,926. Nearly all these processes specify that it is necessary to perform a cleaning treatment between mechanical and electrochemical graining. If this treatment is carried out in an acid medium, only a slight attack on the mechanically produced graining structure is to be expected. The cleaning action is predominant. Contrary to this, an alkaline treatment results in a definite modification of the surface.
A chemical intermediate treatment following the electrochemical, second graining stage and preceding anodization is mentioned in U.S. Pat. No. 2,344,510 and is described in Japanese patent application No. 123,204/78, German Offenlegungsschrift No. 30 12 135 (equivalent to British Pat. No. 2,047,274 or U.S. Pat. No. 4,476,006) and German Offenlegungsschrit No. 30 36 174 (equivalent to British patent application No. 2,060,923). This treatment is carried out either under acidic or alkaline conditions and the above-mentioned effects are attained. Particularly in an alkaline treatment the electrochemical graining structure is, to a large extent, levelled by the removal of thin pore walls, which may have positive effects on the water/ink balance, but entails shorter print runs of the printing plate so produced.
A cathodic surface modification which is carried out after a one-stage electrochemical graining treatment is disclosed in German Offenlegungsschrift No. 32 22 967 (equivalent to U.S. Pat. No. 4,482,444). This treatment obviates the disadvantages of an acidic or alkaline pickling treatment, which are described in this application and leads to support materials with improved water/ink ink balance and reduced abrasion during printing, without adversely influencing the adherence of the photosensitive layer.